1. Field of the Invention
This invention relates to data storage devices and more particularly to a method for enabling read-write storage media to operate with a write once, read many times characteristic.
2. Description of Related Art
Generally a data processing system operates with a host processor including a main memory and a secondary memory comprising one or more magnetic disk storage devices. The magnetic disk storage devices all have a read/write capability. That is, each storage location in the magnetic disk storage device can be altered many times.
There are some applications in which it is necessary or highly advantageous to provide a permanent, non-alterable version of a file. By non-alterable it is meant that the data in the file can not be altered without leaving some sort of audit trail concerning that change. For example, insurance policies, while being written, are subject to change while they are being negotiated or compiled; once the insurance policy is issued, both the insurance company and policy holder want to be sure any change to that policy is detected. Similar requirements for permanence exist for medical records and images.
Given the ease with which data of this type can be altered on conventional magnetic storage media, a number of applications use optical disks for providing such "permanent" or "non-alterable" storage. These optical disks have a write once, read many times characteristic and are called WORM devices. Examples of such optical storage media are disclosed in U.S. Pat. No. 4,689,778 to Miura et al. for an Optical Disc Recording/Reproducing Apparatus with Recorded Track-Section Detector; U.S. Pat. No. 4,760,566 to Kobayashi et al. for a Method of Controlling Write Operation for Rotating Type Recording Medium; U.S. Pat. No. 4,831,611 to Sasaki et al. for an Optical Writing and Reading Method and Apparatus; and U.S. Pat. No. 5,023,854 to Satoh et al. for a Disc Having a Data Read-Only Area and a Data Recording Arm and a Recording Reproducing System Therefor.
The Miura et al. patent discloses a method by which a determination is made concerning the preexistence of data on a track prior to recording information on that track. In essence, Miura et al. use different tracks to determine if data exists in any part of a track and thereafter prevents writing to any track which indicates the presence of preexisting data. Kobayashi et al. disclose a process by which data can be overwritten with or without pre-erasing depending upon the existence of the flag field associated with each block. Sasaki et al. disclose a similar approach wherein overwriting is prevented by writing on a mark recording region. Thereafter overwriting is inhibited. The Satoh et al. patent discloses a read-only type optical disk and related disk drive divided into read-only and read/write areas recorded in the respective identifier portions. This prevents writing to the read-only areas.
Generally such systems transfer data to optical disks in two steps. Data is generated on magnetic disk storage systems. When the data is fixed, it then transfers to the optical disk. This involves multiple transfers.
U.S. Pat. No. 5,233,576 (1993) to Curtis et al. discloses a multi-functional optical disk drive and media using a magneto-optical disk media that can operate as a read/write storage media or as a WORM media and can eliminate transfers from a magnetic disk storage device to an optical disk. The apparatus and methodology disclosed in the Curtis et al. patent is directed to providing a read/write apparatus that has the same level of security as a WORM device. In the apparatus, a media descripter table is contained within a control track. The media also includes a storage state bit within each sector of each track of the media. This storage state bit defines whether the sector is writable or read-only. When the bit is set into the writable state, the sector can be written many times (i.e., operates in a read/write mode). Once the bit is changed to read-only, the sector can no longer be written and cannot be reset to a read/write state. Curtis et al. disclose two locations for each storage bit, namely: (1) the sector mark field area of each sector or (2) the data field area of each sector.
In typical data processing systems, optical disks are stored in changers that must mount each optical disk as it is required. The time required to write optical disks and even to read data from those disks can be up to an order of magnitude longer than corresponding times that characterize magnetic disk storage media. By virtue of the fact that a typical read-only disk can be used only once, once the data on the disk becomes obsolete the optical disk cannot be used again. All these characteristics can increase the "per byte" storage costs beyond the per byte storage costs for magnetic disk storage media.
The Curtis et al. patent does disclose a system that overcomes certain problems. For example, the system disclosed in the Curtis et al. patent eliminates the step of transferring from magnetic media to optical media. However the Curtis et al. patent writing operations require an analysis of data on the optical disk. That is, writing operations to read-only areas produce error messages that are determined only after the retrieval of status resident on the storage media. Such operations, that occur after delays in the millisecond range, can seriously affect system performance. Consequently the general approach has continued to be writing the data to a magnetic storage media and then, upon the data becoming stable, transferring that data to an optical disk.